Particles suitable as carriers for electrophotography

ABSTRACT

Particles (I) which are suitable as carriers for electrophotography consist of 
     a) a magnetic core and 
     b) a shell of alumina, chromium oxide, molybdenum oxide, tungsten oxide, silica, tin oxide or zirconium oxide or a mixture thereof 
     and particles (II) consist of 
     a) a magnetic core and 
     b) a shell of titanium oxide 
     and are obtainable by decomposing a titanium tetraalcoholate in the gas phase by reaction with steam and/or oxygen in the presence of agitated cores, and these particles are used in electrophotographic two-component developers.

The present invention relates to novel particles (I) which are suitableas carriers for electrophotography and consist of

a) a magnetic core and

b) a shell of alumina, chromium oxide, molybdenum oxide, tungsten oxide,silica, tin oxide or zirconium oxide or a mixture thereof.

The present invention also relates to further novel particles (II) whichare suitable as carriers for electrophotography and consist of

a) a magnetic core and

b) a shell of titanium oxide, obtainable by decomposing a titaniumtetraalcoholate in the gas phase by reaction with steam and/or oxygen inthe presence of agitated cores.

The present invention furthermore relates to processes for thepreparation of these particles and their use for the preparation ofelectrophotographic two-component developers, and electrophotographictwo-component developers which contain these particles.

Two-component developers are used in electrophotographic copiers andlaser printers for developing an electrophotographically produced latentimage and usually consist of carrier particles and toner particles. Thecarrier particles are magnetizable particles having sizes of, as a rule,from 20 to 1,000 μm. The toner particles consist essentially of acolor-imparting component and binder and have a size of about 5-30 μm.

In the copying process, the electrostatic, latent image is produced byselective exposure of an electrostatically charged photoconductor rollerto light reflected from the original. In the laser printer, this iseffected by a laser beam.

For the development of the electrostatic image, toner particles aretransported to the photoconductor roller by means of a magnetic brush,i.e. carrier particles oriented along the field lines of a sectormagnet. The toner particles adhere through electrostatic attraction tothe carrier particles and, during transport in the magnetic field,acquire an electrostatic charge opposite to that of the carrierparticles, as a result of friction. The toner particles thus transferredfrom the magnetic brush to the photoconductor roller give a toner imagewhich is then transferred to electrostatically charged paper and fixed.

The carrier particles used have to meet a number of requirements: theyshould be magnetizable and thus permit a rapid build-up of the magneticbrush. Furthermore, their surface should have low conductivity in orderto prevent a short-circuit between the sector magnet and thephotoconductor roller. This conductivity should remain constant overlong operating times of the carrier so that the triboelectric charge ofthe developer can also be kept constant for a long time. Not least, thecarrier particles should also be free-flowing and should not form lumpsin the developer storage vessel.

In order to meet these requirements, the carrier particles consisting ofmagnetically hard or in particular magnetically soft material must as arule be coated.

EP-A-303 918 discloses the coating of steel and ferrite carriers withiron oxide or titanium dioxide which is precipitated by oxidative orhydrolytic decomposition of iron pentacarbonyl or titanium tetrachloridefrom the gas phase onto the carrier particles.

It is also generally known that the surface of the carrier particles canbe coated with polymers, in particular polymeric fluorocarbons, or thesurface of metallic carrier particles can be passivated by oxidation.

However, the last-mentioned coating methods in particular have manydisadvantages. Constant and sufficiently thick layers are difficult toproduce and in addition polymer-coated carriers exhibit poor adhesion ofthe polymer layer to the carrier surface and therefore have only a shortlife.

It is an object of the present invention to provide novel carriers forelectrophotography which possess advantageous performancecharacteristics, and hence to make it possible to achieve optimummatching of the carrier with the particular toner used.

We have found that this object is achieved by particles (I) which aresuitable as carriers for electrophotography and consist of

a) a magnetic core and

b) a shell of alumina, chromium oxide, molybdenum oxide, tungsten oxide,silica, tin oxide or zirconium oxide or a mixture thereof.

We have furthermore found a process for the preparation of the particles(I), wherein volatile aluminum, chlorine, molybdenum, tungsten, silicon,tin and/or zirconium compounds are decomposed by reaction with steamand/or oxygen in the gas phase in the presence of agitated cores.

We have also found novel particles (II) which are suitable as carriersfor electrophotography and consist of

a) a magnetic core and

b) a shell of titanium oxide,

obtainable by decomposing a titanium tetraalcoholate in the gas phase byreaction with steam and/or oxygen in the presence of agitated cores, andthe process, defined thereby, for the preparation of the particles (II).

We have furthermore found the use of the particles (I) and (II) for thepreparation of electrophotographic two-component developers, andelectrophotographic two-component developers which contain theparticles.

The cores of the novel particles (I) and (II) which are suitable ascarriers for electrophotography may consist of the conventionalmagnetically soft materials, such as iron, steel, magnetite, ferrites(for example nickel/zinc, manganese/zinc and barium ferrites), cobaltand nickel, and particles of these metals or metal compounds which areembedded in polymer resins conventionally used for this purpose. Alsosuitable are magnetically hard materials such as strontium ferrite orbarium ferrite or neodymium iron borides.

The cores may additionally be coated with iron oxide and/or titaniumoxide or a mixture thereof in the case of the carriers (I) and with ironoxide in the case of the carriers (II). This type of coating isdescribed in the abovementioned EP-A-303 918.

The novel metal oxide shells of the carrier cores (I) and (II) consistmainly of the following oxides: alumina (Al₂ O₃), chromium(III) oxide(Cr₂ O₃), molybdenum(VI) oxide (MoO₃), tungsten(VI) oxide (WO₃), silica(SiO₂), tin dioxide (SnO₂) and zirconium dioxide (ZrO₂) and, in the caseof the carriers (II), titanium dioxide (TiO₂). Further oxides of themetals in other oxidation states and basic oxides are present as a rulein not more than small amounts, depending on the method of preparation.The oxide shell of the carriers (I) may also consist of mixtures of thestated oxides which have been deposited in succession or simultaneously,and of mixed oxides.

The thickness of the oxide shell is not in itself critical. In principleboth very thin and very thick layers are possible. The optimum thicknessof the oxide shell is dependent on the particular intended use. As arule, it is from about 2 to 500 nm, preferably from 10 to 200 nm.

For the formation of the oxide shell, in the novel processes for thepreparation of the carriers (I) and (II) volatile compounds of thecorresponding metals are decomposed hydrolytically and/or oxidatively inthe gas phase in the presence of the carrier cores to be coated(chemical vapor deposition).

The corresponding carbonyls, halides and alcoholates are preferablyused.

The chlorides are particularly preferred in the case of the halides, butthe bromides and iodides can also be used, for example aluminumtribromide.

The alcoholates may be both aromatic and aliphatic compounds. Forexample, phenolates and benzyl alcoholates and especially C₁ -C₄-alkanolates, such as methanolates, ethanolates, n- and isopropanolatesand n-, tert- and isobutanolates, are particularly preferred here.

Very particularly preferred starting compounds are chromiumhexacarbonyl, molybdenum hexacarbonyl and tungsten hexacarbonyl,aluminum trichloride and silicon tetracholride, tin tetrachloride andzirconium tetrachloride.

In the novel preparation of the carriers (II) coated with titaniumoxide, essentially titanium dioxide, titanium tetraalcoholates, such astitanium tetraphenolate, titanium tetrabenzyl alcoholate and titaniumtetra-C₁ -C₄ -alkanolates, e.g. titanium tetramethanolate, ethanolate,n-propanolate, n-, iso- and tert-butanolate and preferably titaniumtetraisopropanolate, are used.

The decomposition of the carbonyls is preferably effected by oxidationwith oxygen or air, while the halides and alcoholates are preferablydecomposed by hydrolysis with steam in the presence or absence ofoxygen. The alcoholates and halides may also be decomposed oxidatively,but higher temperatures (from about 200° to 600° C.) are required forthis purpose, particularly in the case of the halides. As a rule, onlyheat-stable cores, such as steel or ferrite cores, are thereforesuitable for coating carried out in this manner.

The following process is advantageously used: The carrier cores arefirst fluidized in a heatable reaction vessel, preferably in an agitatedfixed bed or a fluidized bed, by means of an inert gas, such asnitrogen, and are heated to, as a rule, from 100° to 400° C. preferablyfrom 200° to 300° C. The vaporized metal compound as a mixture with aninert gas, such as nitrogen, and the particular reactant, either air oranother oxygen/nitrogen mixture for oxidation, or steam with a carriergas, such as nitrogen or air, for hydrolysis are then fed in separately.The concentration of oxygen, steam and especially the metal compound inthe particular carrier gas should preferably be less than about 5% byvolume in order to ensure uniform coating of the carrier surface withmetal oxide.

The thickness of the metal oxide layer formed depends of course on themetal compound fed in and can thus be controlled via the coating time.

After cooling, the product can then be discharged and can be usedwithout further aftertreatment.

Coating of the carrier cores by means of the gas phase decomposition ofcorresponding metal compounds is the preferred procedure for thepreparation of the novel carriers. In principle, however, this can alsobe effected by precipitating the metal oxide or hydroxide from anaqueous metal salt solution or from an organic solvent, followed by theheat treatment.

The novel carriers have homogeneous, abrasion-resistant metal oxidelayers. Their surface has the desired low conductivity. Depending on theparticular toner used, they permit both a positive and a negative tonercharge and can therefore be specifically selected for the intended use.Moreover, they have a long life and can therefore generally beadvantageously used with the commercial toners for the preparation ofelectrophotographic two-component developers.

EXAMPLES

A. Preparation of novel carriers

The crude carriers were coated in an agitated fixed bed. The reactionvessel used was a 500 ml quartz flask having a diameter of 10 cm and wasfastened to a rotary evaporator. A thermostatable metal nozzle whichcontained two separate water-cooled gas inlet tubes and a thermocouplewith a gas-tight seal was introduced through the motor shaft of therotary evaporator into the center of the carrier bed in the flask. Thequartz flask was heated by means of a 6 l heating jacket. The metalcompound vaporized in an evaporator vessel upstream of the nozzle wasfed, in a stream of nitrogen, through an inlet tube. The second inlettube was used for the introduction of nitrogen and of air for oxidationor of air laden with steam in a further upstream evaporator vessel.

In the apparatus described above, x kg of the crude carrier

A: spherical steel carrier having a mean particle size of from 75 to 180μm, type TC 100 (Pometon S.p.A., Italy),

B: ferrite carrier having a mean particle size of from 45 to 105 μm,type KBN 100 (Hitachi, Japan) or

C: ferrite carrier having a mean particle size of from 20 to 60 μm, CM30-60 SH (Ho/ gana/ s, Sweden) were heated to 250° C. at 50 rpm in astream of 40 l/h of nitrogen. y g (ml) of metal compound in a stream ofn l/h of nitrogen were passed into the apparatus in d h via theevaporator vessel heated to the evaporation temperature V [° C.]. Inaddition, s l/h of air for oxidation or, via the second evaporatorvessel heated to 20° C., steamladen air (w l/h) for hydrolysis wereadditionally fed in.

The carrier coated in this manner was then cooled under a stream of 50l/h of nitrogen and was discharged.

Details of the experiments and their results are summarized in Table 1.

                                      TABLE 1    __________________________________________________________________________    Exam-        x kg of               y g of metal                       Evaporation tempera-                                  n 1/h of                                       d h Evapora-                                              s 1/h                                                  w 1/h of                                                       Metal content of                                                       coated    ple crude carrier               compound                       ture V [°C.]                                  nitrogen                                       tion time                                              of air                                                  water/air                                                       carrier % by    __________________________________________________________________________                                                       weight    1   1.8           A   10 W(CO).sub.6                       80         50   20     50  --   W: 0.20    2   1.8           A   10 Cr(CO).sub.6                       80         40   22     50  --   Cr: 0.17    3   1.8           A   10 Mo(CO).sub.6                       80         50   15     50  --   Mo: 0.14    4   1.5           A   10*                  SiCl.sub.4                       -40        20   7      --  20   Si: 0.59    5   1.8           A    6.5*                  SnCl.sub.4                       9          10   12     --  10   Sn: 0.05    6   1.5           A    3.5                  AlCl.sub.3                       150        100  7      --  10   Al: 0.05    7   1.0           B   10 W(CO).sub.6                       80         50   20     50  --   W: 0.47    8   1,0           B   10 Cr(CO).sub.6                       80         40   22     50  --   Cr: 0.22    9   1.0           B   10 Mo(CO).sub.6                       80         50   15     50  --   Mo: 0.38    10  1.0           B   10*                  SiCl.sub.4                       -40        20   7      --  20   Si: 0.08    11  1.0           B    6.5*                  SnCl.sub.4                       9          10   12     --  10   Sn: 0.01    12  1.0           B    3.5                  AlCl.sub.3                       150        100  7      --  10   Al: 0.06    13  1.0           C    7*                  Ti(i-                       170        20   2      --   50**                                                       Ti: 0.10                  OC.sub.3 H.sub.7).sub.4    14  1.8           A    1.4                  Mo(CO).sub.6                       80         80   2      50  --   Mo: 0.02    __________________________________________________________________________     * = ml     ** = Water heated to 40° C.

B. Measurement of the electrical resistance and of the electrostaticcharge capacity of novel carriers

B.1. Electrical resistance

The electrical resistance of the carriers from Examples 1 to 14 ismeasured using the C meter from PES Laboratorium (Dr. R. Epping,Neufahrn). For this purpose, the carrier particles were agitated for 30s in a magnetic field of 900 Gauβ at a voltage U₀ of 100 V (capacitanceC=1 nF).

The resistance R can be calculated according to the following formulafrom the decrease of the voltage with time after the applied electricfield has been switched off:

    R=t/[C/ln (U.sub.0 /U)]

where R is the resistance [ohm],

t is time of the measurement [s],

C is the capacitance [F],

U₀ is the voltage at the beginning of the measurement [V]and

U is the voltage at the end of the measurement [V].

The resistance R is usually stated as logarithmic values. The results ofthe measurement are shown in Table 2.

B.2. Electrostatic charge capacity Q/M

The electrostatic charge capacity Q/M of the carriers from Examples 1 to14 was determined against the following toners:

T1: Positively chargeable toner for the commercial Siemens ND 2/3 laserprinter

T2: Negatively chargeable toner for the commercial IBM 3827 laserprinter;

T3: Neutral toner without pigment and further additives: styrene/butylacrylate resin (Neocryl.sup. B 1062 toner resin; Polyvinylchemie, TheNetherlands) milled in a laboratory pinned-disk mill to a mean particlesize of 26.7 μm and sieved to give a fraction less than 36 μm.

For this purpose, the carrier particles were first mixed with theparticular toner in a weight ratio of 98.5:1.5 and shaken in a glassvessel for 2 minutes. A weighed amount of this mixture was thenintroduced to a hard blow-off cell coupled to an electrometer (Q/M meterfrom PES Laboratorium, Dr. R. Epping, Neufahrn). The mesh size of thesieves used in the cell was 40 μm and was chosen so that no carrierswere discharged but the toner powder could be completely blown off. Whenblowing off and extraction of the toner were complete, the charge wasdetermined and was related to the weight of the blown-off toner byreweighing.

The results of the measurement are summarized in Table 2.

                  TABLE 2    ______________________________________    Electrical resis- Electrostatic charge    tance, expressed as                      capacity Q/M [μC/g]    Example log R [log ohm]                          T1       T2     T3    ______________________________________    1       9.42          +20.4    -5.9   +23.4    2       8.12          +22.1    -12.0  -18.8    3       8.21          +20.1    +13.3  +24.9    4       9.60          +11.5    -4.3   -1.0    5       9.41          +44.3    -1.0   -1.2    6       9.90          +22.4    -9.7   -2.2    7       10.58         +15.2    +3.6   -1.9    8       8.42          +11.8    -3.0   0    9       9.53          +15.1    +6.0   +2.0    10      10.22         +7.5     +3.8   +2.9    11      10.47         +8.5     +5.0   +1.8    12      10.40         +14.2    -0.7   0    13      10.90         +10.7    +1.0   0    14      8.15          +25.4    -7.1   +0.9    ______________________________________

We claim:
 1. Carrier particles for electrophotography consisting ofa) amagnetic core and b) a shell consisting of molybdenum oxide, tungstenoxide or a mixture thereof.
 2. Carrier particles as claimed in claim 1,wherein the shell is molybdenum oxide.
 3. Carrier particles as claimedin claim 1, wherein the shell is tungsten oxide.
 4. Carrier particles asclaimed in claim 1, wherein the magnetic core is iron, steel, magnetite,ferrite, cobalt, nickel or neodymium iron boride.
 5. Anelectrophotographic two-component developer, comprising carrierparticles consisting ofa) a magnetic core and b) a shell consisting ofmolybdenum oxide, tungsten oxide or tin oxide or a mixture thereof,andtoner particles.
 6. The two-component developer as claimed in claim 5,wherein the shell of the carrier particles is molybdenum oxide.
 7. Thetwo-component developer as claimed in claim 5, wherein the shell of thecarrier particles is tungsten oxide.
 8. The two-component developer asclaimed in claim 5, wherein the shell of the carrier particles is tinoxide.
 9. A process for the preparation of carrier particles forelectrophotography consisting ofa) a magnetic core and b) a shellconsisting of molybdenum oxide, tungsten oxide or a mixture thereof,consisting of decomposing a volatile metal compound selected from thegroup consisting of compounds of molybdenum, compounds of tungsten and amixture thereof by reaction with steam or oxygen or both in the gasphase in the presence of heated, agitated magnetic core particles tothereby obtain said carrier particles.
 10. A process as claimed in claim9, wherein the volatile metal compounds are metal halides or metalcarbonyls.